1. Field of the Invention
This disclosure relates generally to tomographic imaging, and in particular to attenuation correction for gamma ray detection in a gamma camera.
2. Introduction
In conventional gamma cameras such as Anger cameras known in the art, a single radiation detector having a planar surface is employed for detecting gamma rays for tomographic imaging. A radiopharmaceutical or radioisotope (for example, Tc-99m or Tl-201), chosen for its affinity for a particular region of the body, is administered to the patient. The radiopharmaceutical or radioisotope emits gamma rays in all directions that are detected by gamma cameras.
The system accumulates counts of gamma photons that are absorbed by a crystal in the gamma camera detectors, usually a large flat crystal of sodium iodide with thallium doping in a light-sealed housing. The crystal scintillates in response to incident gamma radiation: when the energy of an absorbed gamma photon is released, a faint flash of light is produced. This phenomenon is similar to the photoelectric effect. Photomultiplier tubes (PMT) behind the crystal detect the fluorescent flashes and convert them into electrical signals, and a computer sums the fluorescent counts. The computer in turn constructs and displays a two dimensional image of the relative spatial count density or distribution on a monitor. This image then reflects the distribution and relative concentration of radioactive tracer elements present in the organs and tissues imaged.
Between the gamma camera detector and the emission source is usually a collimator. The collimator's function is to allow only gamma rays that are traveling perpendicular to the surface of the detector to reach the detector. The collimator is constructed of a dense, high-atomic-number material, such as lead. The material is bored with numerous tiny straight holes, which allow parallel gamma rays to pass through. If the gamma ray is not traveling along the path of the hole then it will be absorbed by the material and will not reach the detector. It is for this reason that the direction the detector faces is important.
In order to obtain spatial information about the gamma emissions from an imaging object a method of correlating the detected photons with their point of origin is required. Single Photon Emission Computed Tomography (SPECT) captures multiple images from multiple angles in order to make a three-dimensional representation of the object under observation. The area to be imaged is generally called the reconstruction area, since the data gathered by the gamma detectors is used to reconstruct an image of the object. The object to be imaged is located inside of the reconstruction area.
One issue with SPECT imaging is the fact that as the gamma rays travel through the body, they become attenuated. A significant portion of the emitted photons are obstructed from reaching the detectors by colliding with atoms. When this occurs, one significant possibility is a course alteration away from the detector that may result in a missed detection. The degree of attenuation depends upon the amount and density of matter between the emitting source and the detector, and will vary from subject to subject according to body composition. The more attenuation present, the more probable will be an inaccurate reconstruction of image data.
Unless the amount of attenuation is known, the detected activity within a defined energy window underestimates the true activity. This results in poorer contrast and attenuation artifacts in the reconstructed images. Conditions such as these reduce the confidence one may have in extracting information for diagnosis.
One method to compensate for this attenuation is by approximating the amount of attenuation based on the tissue depth of the emitting source and using estimated attenuation values for certain tissues. This approach is easy to perform, but it is only an approximation and does not provide a very accurate attenuation factor. To get a more accurate attenuation factor, one must use a known transmission source from outside the body, such as an x-ray or gamma source (such as Gd-153) to transmit external radiation through the reconstruction area and then measure the attenuation of the transmitted source by the reconstruction area.
It is known to measure the actual attenuation coefficients of body tissues by placing a line source of gamma radiation on one side of the body and measuring the transmission of the gamma radiation through the body as a function of direction, i.e. collecting transmission CT data, as the line source is scanned across the patient's body. See, e.g. U.S. Pat. No. 5,576,545 (Stoub et al.) incorporated herein by reference in its entirety.
The traditional method for SPECT used large fixed-angle detectors attached to a stationary gantry. The detector was not able to alter its viewing angle independent of its movement along the gantry. The gantry provides the detectors with the structural support needed to orbit about the patient. The orbiting of the detectors is necessary in order to obtain enough information to accurately reconstruct a three dimensional image of the patient or object being observed.
Another more modern approach to tomographic imaging uses multiple smaller, articulating detectors that are able to swivel or pivot, so that the detector face can sweep across a larger area at each reconstruction angle. See, e.g., U.S. Pat. No. 5,757,006 to DeVito et al., and U.S. Pat. No. 6,242,743 to DeVito et al. Such articulating detectors can thus acquire projection data from areas outside the plane of the gantry. Some detectors combine the two elements so that articulating detectors are also able to move linearly to further enhance the viewing angle of the detector. Further, approaches such as disclosed in the '743 patent provide multiple small, articulating detectors completely surrounding a patient such that no orbiting of the detectors about the patient is necessary, as all the multi-angle information may be obtained simultaneously.
The prior art approaches to attenuation correction, such as, e.g., fan-beam collimated sources, scanning sources, and profile typing, have been developed for the traditional, large fixed-angle detectors with stationary gantries. These approaches are not compatible with the modern, small articulating multiple detector approach. The present invention improves upon existing prior art by using a system that can determine an accurate attenuation factor that can be used with both the traditional and modern approaches to SPECT imaging.